JPH0757405B2 - Thread forming screw - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates generally to threaded fasteners, and more particularly to threaded screws having tapered ends and apparatus and methods for making the threaded screws. There is.

[0002]

BACKGROUND OF THE INVENTION It is well known to use thread forming screws to securely connect objects. This type of screw is inserted into an unthreaded hole or hole and, when torque is applied, forms a thread in the hole or hole. Such a screw structure is highly vibration resistant in order to maintain the connection between the connected bodies even when the connected bodies are subjected to considerable stress. Some self-threading screws include threads with multiple lobes around the periphery of the shank of the screw. The lobes, or corrugations, are more resistant to screw loosening. Significant torque is required to form the threads of the screw shank. The greater the torque required, the greater the required strength of the screw. In addition to providing screws made of stronger materials, high torque in the threading process adds cost to the final product. Therefore, it is required to form a screw thread with a minimum torque. Tapered threads present additional difficulties in providing a tapered section with a ridged or well-formed thread. The unraveled threads not only reduce the extraction torque required to remove the screw, but also make the screw insertion more difficult. Thread formation The size, shape and spacing of the threads at the tapered end of a screw is a very important issue for easy insertion of a screw into an unthreaded hole or hole. It is this part of the screw that initiates the threading process.

SUMMARY OF THE INVENTION The present invention provides a tapered screw thread to provide a well-developed tapered thread at the end of the blank with improved stress relief to facilitate the tapering and threading operations. The above advantages can be provided in a thread forming screw formed from a cylindrical piece of material that passes through a thread rolling die.

[0004]

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved thread forming screw which is characterized by a low advance / extract ratio. Another object of the present invention is to
The purpose of the thread forming operation and insertion into the work piece is to provide a self-tapping screw with minimal torque. Yet another object of the present invention is to provide a thread forming screw having an improved tapered thread which facilitates insertion of the screw into an unthreaded work piece, i.e. a work piece. That is. Another object of the present invention is to provide a thread forming screw which is inexpensive and can be manufactured relatively easily, and which is easily threaded to easily form a thread on a workpiece, and therefore may have a small strength. That is. Yet another object of the present invention is to provide a thread forming screw having a fully ridged or sharp edged thread along its entire length, including tapered ends. Yet another object of the present invention is to maintain the thread forming screw in alignment with the cylindrical hole into which the screw is inserted to facilitate threading and entry of the screw into the hole. .

A thread forming screw comprising an elongated shank having a continuous male thread and a plurality of outer lobes circumferentially distributed about the shank so as to be equally spaced, wherein the male thread is The crest extends along the shank in a first spiral direction, the lobe has an outer lobe extending along the shank in a second spiral direction opposite the first spiral direction, and the shank comprises: Having an inner solid core portion in the form of a circle in any plane perpendicular to the axis of the shank, the lobe projecting outward beyond the core portion, and the shank having a cylindrical threaded portion. , A tapered threaded end portion, wherein the lobes are disposed on the cylindrical threaded portion and the tapered threaded end portion, and have a fully threaded thread. The cylindrical and tapered threaded portion is a non-tapered, non-tapered cylindrical material to enhance stress relaxation during formation and reduce the torque required during thread formation. The object of the invention is achieved and the drawbacks of the prior art overcome by a thread-forming screw, characterized in that the tapered threaded end portion formed from a piece comprises four pitches. .

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a side view of a thread forming screw 10 in accordance with the present invention. The screw 10 includes a head 12 and an elongated shank 14. Shank 1
4 has external threads 16 with spaced lobes 18, as more clearly shown in the cross-sectional view of FIG. 2 taken along line 2-2 of FIG. Robe 18 is shown in FIG.
As shown in FIG. 3, the shank 14 is distributed in the peripheral direction around the axis of the shank 14, and extends spirally along the axis of the shank 14 in the spiral direction opposite to the spiral direction of the thread 16. The shank 14 of the thread forming screw 10 has a straight cylindrical portion 14
a and a tapered end 14b. In every plane perpendicular to the axis of the shank 14, the shank 14 has an inner core portion 20, which is of circular shape and is concentric with the axis of the shank 14, It is inside the two-dot chain line circle 21 shown in FIG.
The lobes 18 project from the core portion 20 and
The core portion 20 constitutes a central shank portion inside a circle 21 that is coaxial with the axis of the shank 14.

When the screw is screwed into the workpiece, the screw is
It is rotated in the direction of arrow 22 in FIG. Each lobe 18
It has an elongated front leg 24 which develops in the direction of rotation 22 gradually away from the shank axis. Each robe 18
Further has a rear leg 26, which is bent sharply in the direction of the shank axis,
Much shorter than Therefore, the lobe 18 is asymmetric. As shown in FIG. 2, each of the elongated front legs 24 is convexly bent into a smooth rear leg 26, which is concave only in its innermost region, at which position the rear leg 26 is Adjacent to the circular core portion 20 and the front leg of the next lobe 18. Each front leg 2
The development of the four shank away from the axis is continuous from the beginning of the front leg to the confluence with the sharply bent rear leg 26. Furthermore, the thread 16 has its top 28
It has a uniform radial width between the bottom and the valley bottom 30.
As such, the morphology of the lobes 18 and crests 28 of the threads 16 is consistent with the lobe morphology along the roots 30 of the threads. In the preferred embodiment, three lobes 18 are evenly distributed about the shank axis, each lobe extending over an arc of about 120 °, as shown in FIG. The thread forming screw 10, which is formed by a thread rolling die, has a helical lobe so that the same pressure is applied over the entire length of the screw to minimize slippage during thread rolling. Furthermore, as in the case where the lobes simply extend in the axial direction, the irregularities in the rolling of the lobes (interru
Since there is no ption), the roundness of the finished screw is maintained. As such, a single rolling operation can be used to form the threads and lobes of the shank 14 of the thread forming screw 10.

Each lobe 18 of the thread forming screw 10 gradually and continuously rises up to a maximum distance from the axis of the shank and then descends sharply before the slower rise forming the next lobe. Return to the axis. Therefore, each lobe gradually develops away from the shank axis in the direction in which the screw is screwed into the workpiece. The shorter rear leg 2 of each lobe 18 when screwing back in from the threaded hole
The steep heel formed by 6 provides resistance to screw disengagement because the interlocking threaded metal squeezes behind each rear leg during thread formation. Since the threads rise and fall according to the lobe configuration, less torque is required than with three lobes, each of symmetrical construction, such as in other thread forming screws. Since it does not, the thread forming operation can be performed in a much easier way.
The unique three-lobe structure of the thread forming screw 10 of the present invention is
Allows the screw to form a thread. The thread forming operation is greatly facilitated by the unique construction of the lobes mentioned above. Since the lobes are spirally arranged around the shank axis, the thread formation in the workpiece into which the screw 10 is screwed can be smoothly performed without any irregularity, and thus the thread formation operation is further promoted. Let Therefore,
The thread forming screw 10 is particularly suitable as a tapping device and the term "thread forming screw" is intended to include such a tapping device having the characteristic lobe structure described above. Is.

Screws of the type described thus far perform much better in standard "pull-out" tests. This improved performance is due to the lobes located in a spiral and the high points of each lobe located outside the screw 360 ° around the shank so that the object to be extracted engages the mating member 360 °. It is due to. As such, extraction of the thread forming screw 10 according to the present invention requires much more force than extraction of any other type of lobed screw having lobes distributed along the axis of the shank. After using the thread forming screw 10 to form a thread in a hole in a workpiece into which the screw is screwed,
The lobes 18 are positioned to "wipe" in the thread and, at the same time, spirally form a thread tapped around 360 ° of the shank, resulting in a particularly low torque. Referring to FIG. 3, a device for forming threads and lobes on the shank 48 is shown in simplified schematic form. In operation, the two dies 32a and 3
2b are positioned such that their work surfaces face parallel to each other and are spaced apart by an amount sufficient to accommodate the shank 48 of the blank forming the thread forming screw. The upper die 32a remains stationary while the lower die 3
2b are displaced in the plane of their work planes, as indicated by the direction of arrow F in FIG.
8 rotates clockwise as indicated by arrow C. During the initial part of the thread rolling operation, the dies 32a and 32
The grooves and ridges in b form threads around the shank 48 in the usual manner. However, the corrugations of the dies 32a, 32b, described below, then engage the shank 48 to form lobes in the threads of the screw.

Referring to FIG. 4, the additional details of the rolling of the three lobe threaded blanks 40 between the two threaded dies 42 and 43 are stylized in cross-section. Indicated by. It should be emphasized that FIG. 4 is stylized and does not accurately represent the actual thread formation. Each of the threaded material pieces 40 includes a screw top portion 44 and a screw root portion 46 which are shown by solid lines. Similarly, dies 42 and 43 include threaded tops 48 and grooves 50, also shown in solid lines. If the cross-section is accurate, the thread groove 46 of the blank 40 and the grooves of the dies 42 and 43 would be indicated by dotted lines. In the actual cross-sectional view, the material piece 40 is spirally lowered and overlaps itself at any point, and the lobes are also spirally shaped, so that the same state may be obtained. . These discrepancies simplify the drawing and reduce the dies 42 and 43.
Was provided to help visualize the rolling action of the blank piece 40 between. As described above with reference to FIG. 3, the blank 4
During the 0 roll, the upper die 42 moves to the right, as indicated by the direction of arrow A, while the lower die 43 remains stationary. This, of course, is the relative leftward movement of the lower die 43 with respect to the upper die 42, as indicated by the direction of arrow B in the figure. This relative die movement causes the blank 40 to rotate clockwise about axis O in the direction of arrow C. During this rotation, the thread crests 44 of the blank contact the die grooves 50 and the thread troughs 46 of the blank contact the ridges 48 of the die. Dies 42 and 43
Remain at regular intervals and the axis O of the blank 40
Is centrally located between the dies. Thus, the rotation of the blank in contact with both dies resembles the rolling of a flanged vehicle on a truck. If the point of rotation with respect to the lower die 43 is at the point of contact between the thread troughs 46 of the blank and the corresponding die ridges 48, the screw crests 44 correspond, as indicated by the direction of arrow D. It should slide backwards relative to the die groove 50. On the other hand, if the point of rotation is at the thread crest 44 of the blank (and at the bottom of the die groove 50), the thread trough 46 is slid forward as indicated by arrow E. A similar effect occurs when the blank piece 40 engages the upper die 42. Similar slippage occurs at both the top and the valley for any point of rotation between the top and the valley. Because of these slips, a given distance along the die in any plane other than the plane in which the actual rotation occurs does not match the same distance along the rotating threads. This is less effective in a continuous thread configuration, but is effective in forming irregularities, or lobes, in the disc. In the present example, the die configuration has pressing, which is described in detail below, and the spacing L between subsequent pressings is such that this spacing is the perimeter of the blank 40 measured in the plane of actual rotation. Otherwise, it will not be aligned with the corresponding lobe configuration 54 of the blank. Therefore, the distance between successive lobe forming pressings along the die is determined by the pitch circle diameter of the screw and the press portion diameter,
Equal to the peripheral distance between successive lobes formed on the screw measured along a cylindrical surface having a diameter approximately midway between the outer diameter.

Referring to FIGS. 6 and 7, a partial perspective view and an elevation view, respectively, of a die 60 used to form a thread forming screw in accordance with the principles of the present invention are shown. Figure 5
FIG. 8 is an enlarged partial sectional view of the upper portion of the die 60 shown in FIG. 7 taken along the line 5-5 in FIG. 7. Thread forming surface 5 of die 60
8 has a ridge 62 and a groove 64 that continuously extend toward the right or leading end of the die. However, toward the left or end of the die, the ridges 62 and grooves 64 that make up the thread forming surface 58 are irregular or wavy with respect to the plane of the work surface of the die. These corrugations are arranged in diagonal rows, as indicated by the irregular line 68. Note that these rows extend at a row angle with respect to the top and bottom surfaces of the die and with respect to the path of motion of the die. This angle is the same as the twist angle of the lobe helix indicated by line 18 in FIG. It should also be noted that the ridges 62 and grooves 64 extend at a helix angle α with respect to the upper and lower surfaces of the die and the path of motion of the die.
This angle α is opposite in direction to the twist angle, as shown in FIG.
It is the same as the twist angle of the thread. Referring to FIG. 6, the corrugations on the thread forming surface 58 of the die 60 are actually a series of “pressing” such that the ridges 62 and grooves 64 are actually cross sections, contours or helix angles. It is slightly inclined or inclined with no significant change in α. The pressing depth 66, that is, the distance from the highest point to the lowest point of each pressing ridge 62 or groove 64 measured in the direction perpendicular to the plane of the thread forming surface 58 depends on the thread size. Change. Each pressing 66 is asymmetric, that is, each pressing gradually bulges outward with respect to the plane of the thread forming surface 58 in a direction to the left over most of its length, and then either It should also be noted that, for example, it returns abruptly inward with respect to the plane of the work surface. This configuration creates threads with asymmetric lobes as described above with respect to FIGS. The dies spaced with respect to die 60 are identical in size and shape to those of die 60, but form threads that are reversed with respect to column angle and twist angle α to complement die 60. It should also be understood that it has ridges and grooves, and corrugations or pressings that form lobes.

As shown in FIG. 5, the pitch P is the distance between two corresponding points on adjacent threads, that is, the distance the screw travels in one revolution. The distance P / 2 is half the pitch, ie the distance between adjacent ridges 62 or adjacent grooves 64. The plane of the pitch circle diameter is the plane that passes through the middle between the parallel arrays of ridges 62 and grooves 64, while the pitch circle diameter D is the plane of the aligned ridges and grooves in the direction transverse to the plane of the pitch circle diameter. Is the distance between. In accordance with the principles of the present invention, the thread forming surface 58 of the die 60 includes a straight portion 60a and a tapered portion 60b.
The taper portion in the preferred embodiment includes a combination of ridges 62 and grooves 64 that form the four lowest pitch or lowest threads. Referring to FIG. 8, there is shown a cross-sectional view of a prior art arrangement for forming threads in a thread forming screw 74 having a shank 76. A pair of dies 70 having alternating ridges and grooves and facing surfaces with pressing as previously described.
And 72 are positioned for interference fit engagement with the shank 76 of the blank 74. The shank 76 includes a cylindrical straight portion having a length S and a tapered end having a length T. Similarly, the first and second dies 7
The facing faces forming the 0,72 thread include alternating ridges and grooves and a straight top with a length GL. The ridged and grooved straight facing surfaces of the first and second dies 70 and 72 have a length TL in a portion of the second adjacent first and second dies 70 and 72. It has a taper that includes three pitches. here,
It should be noted that FIGS. 8 and 9 show the thread forming die and the thread blank in place prior to thread forming.

In the inventive thread-forming configuration of FIG. 9, the first and second dies 78 and 80 are similarly
It includes straight ridges and groove portions each having a length GL. Immediately below the facing straight portions of the first and second dies 78 and 80 are tapered portions each having a length TL. In the thread forming according to the present invention, the shank 84 of the blank 82 is a straight cylindrical shape having a length S'and does not include a tapered end. In the preferred embodiment, the lower tapered facing surfaces of the first and second dies 78, 80 include four pitches. It should be noted here that S + T = S '. Furthermore, in the prior art approach of FIG. 8, the body of shank 76 (non-tapered portion).
Does not extend into the tapered (ie, raised) portion of dies 70 and 72. On the other hand, in the inventive thread forming approach of FIG. 9, the cylindrical shank 84 of the blank 82 extends into the tapered portion of the dies 78 and 80. 10 and 11, material pieces 74 and 82 after the thread forming operation are shown, respectively. From FIG. 10, the shank 7 of the prior art piece of material 74
The tapered end of 6 is extended downwards and has an increased length T '.
It turns out that you have. The extended tapered end of shank 76 of prior art blank 74 extends beyond three pitches and the shank length (S + T ') is the length of the straight portion of thread forming dies 70 and 76. It is equal to the sum of the length TL of GL and the tapered portion.

11, the shank 84 of the blank 82 is also extended to a length S "during the thread forming operation. Further, the end of the shank 84 has a first end. And four pitches on the opposing thread forming surfaces of the second dies 78 and 80. In the preferred embodiment, the shank 8 of the blank 82 is tapered.
4 has four pitches of threads, at which the inner diameter of the end of the shank 84 remains constant and the outer diameter decreases toward the end. There is a pitch followed by two end pitches where the outer diameter remains constant. Two end pitches of the same diameter keep the screw in alignment with the cylindrical hole or hole into which the screw is inserted, ensuring that the screw enters straight into the hole. Thus, a thread forming screw and method of making the same have been shown. The method includes inserting a non-tapered cylindrical shank of a thread blank between the opposed thread forming surfaces of a pair of rolling dies. The facing faces of the rolling die are provided with pressings located at a plurality of intervals. In this pressing,
The ridges and grooves of the thread forming surface are slightly tilted or inclined with respect to the plane of the thread forming surface without any significant change in cross section, contour or helix angle. These pressings form a plurality of spaced lobes around the shank threads, in which case the pressing is opposite to the direction of the alternating ridges and grooves on the die threading surface. Direction. The facing tapered portions of the die shape the ends of the cylindrical blank into a threaded, tapered form as follows. The tapered threads on the end of the blank include the four pitches of the threads,
At these pitches, the inner diameter remains constant,
On the other hand, the outer diameter continues to decrease sharply for the two pitches but remains constant at the reduced diameter for the last two pitches. The last two smaller and constant diameter pitches keep the screw in alignment with the hole into which it is inserted to facilitate its entry into the hole. The cylindrical end of the unthreaded piece of material provides increased material to form a fully ridged thread in the tapered thread forming location, while the die and screw taper Increasing the length of the broken portion to four pitches increases the stress relaxation, thus reducing the force required for thread formation and maintaining a high extraction force while maintaining the screw It is easier to get inside.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that variations and modifications can be made in the broader features of the invention without departing from it. Will. Therefore, it is the intention of the appended claims to cover all such variations and modifications that are within the true spirit and scope of this invention. The matters described in the above description and the accompanying drawings are
It is presented by way of example only and not as a limitation. The actual scope of the invention is intended to be defined in the claims herein, when considered in the proper position based on the claims.

[Brief description of drawings]

FIG. 1 is a side view of a thread forming screw according to the principles of the present invention.

2 is a cross-sectional view of the thread forming screw of FIG. 1 taken along the line 2-2 of FIG.

FIG. 3 is a simplified schematic diagram showing a thread rolling operation in forming a thread forming screw.

FIG. 4 is a simplified schematic diagram showing additional details of a thread and lobe forming operation with a pair of rolling dies to form a thread forming screw as in the present invention.

FIG. 5 is a partial cross-sectional view of a portion of a die used to form threads according to the present invention.

FIG. 6 is a partial perspective view showing a thread forming portion and a lobe forming portion of a die used for forming a screw according to the present invention.

7 is an elevational view of the thread forming die of FIG.

FIG. 8 is a simplified cross-sectional view showing a prior art screw in place between thread forming dies prior to the start of the thread forming operation.

FIG. 9 is a simplified cross-sectional view of a screw showing a screw made in accordance with the present invention in place between thread forming dies prior to the start of the thread forming operation.

FIG. 10 is a simplified cross-sectional view of a prior art screw shown in place between a pair of dies forming threads on a thread blank.

FIG. 11 is a simplified cross-sectional view showing a prior art screw according to the present invention in place between a pair of dies forming threads on a thread blank.

[Explanation of symbols]

 10 ... Thread forming screw 14 ... Shank 14a ... Straight portion of shank 14b ... Tapered portion of shank 16 ... Thread 18 ... Lobe

Claims (3)

[Claims] 1. A thread forming screw comprising an elongated shank having continuous male threads and a plurality of outer lobes circumferentially distributed about the shank so as to be equidistantly spaced. The male thread extends in a first spiral direction along the shank and the lobe has an outer lobe extending in a second spiral direction opposite the first spiral direction along the shank, the shank Has an inner solid core portion in the form of a circle in any plane perpendicular to the axis of the shank, the lobe protruding outward beyond the core portion, and the shank having a cylindrical thread cut. A portion and a tapered threaded end portion, wherein the lobes are located on the cylindrical threaded portion and the tapered threaded end portion and are fully raised. Screw thread Stress relaxation in the formation of
In order to reduce the torque required during thread formation, the cylindrical and tapered threaded portion is formed from a non-tapered, non-tapered cylindrical piece of material, A thread forming screw characterized in that the threaded end portion which has become four includes four pitches. 2. The tapered threaded end portion comprises:
It has a constant inner diameter and includes four pitches, two of the four pitches have outer diameters that decrease towards the end, the other two pitches are screws, holes for inserting the screws. 2. The thread forming screw of claim 1, wherein the thread forming screw has a constant outer diameter to maintain alignment and facilitate the screw entry into the hole. 3. The threads are formed on an elongated shank of the screw by a pair of opposed rolling dies that engage the shank, each of the rolling dies having a straight portion and a tapered portion. Including, the straight portion and the tapered portion of the rolling die are arranged in a facing relationship with each other, the length of the material piece is a cylindrical shape without the threading and not tapering. 3. The thread forming screw of claim 2, wherein the ends of the blank are of such length that they extend into the opposed tapered portions of the rolling die.